As discussed in International Patent Application No. WO 94/19042, it is known that the site of deposition within the respiratory tract of an inhalable substance can be influenced by the level of electrostatic charge on the particles of the inhalable substance. International Patent Application No. WO 94/19042 discloses a device in which the substance to be administered is dispensed in the form of inhalable particles into a passageway defined by a mouthpiece of the device and an arrangement of electrodes within the passageway is used to impart electrostatic charge to the particles so dispensed. In this way, the electrostatic charge characteristically imparted to particles on being dispensed from a particular type of dispensing means can be modified in a controlled manner as they pass through a charging region established by the electrode arrangement. Such modification is stated to encompass increases, reductions, reversal and neutralisation of the level of electrostatic charge on the particles.
With such an arrangement, it is difficult to secure a uniform level of electrostatic charge on the particles since the particles are dispersed into the inhaled airstream and passed through the electric field developed by the electrodes. Particles at different locations in the airstream therefore tend to receive difficult levels of electrostatic charge resulting in particles with a wide spectrum of electrostatic charge.
An inhaler is also known from EP-A-234842 in which a liquid spray is produced in such a way that the droplets inhaled are electrically discharged prior to inhalation. In this form of inhaler, the liquid is sprayed from a spraying edge by developing an intense electric field between the spraying edge and a shield electrode so that the liquid is drawn into a ligament which then breaks up into electrostatically charged spray droplets. The charge on the droplets is then neutralised prior to inhalation by means of a sharp discharge electrode.